Motivational psychologist: This is why you're not achieving your goals

It hasn't even been two months since New Years, but you've probably already forgotten your 2017 resolutions.

According to a psychologist who studies motivation, you don't fail to act on your goals simply because you're not determined enough.

"Everything we do with time and money is a trade-off," says Dan Ariely, professor of psychology and behavioral economics at Duke University and TED speaker.

"The problem is we don't always see what we're giving up," he tells CNBC.

To make smarter time decisions, Ariely suggests adopting an old-fashion organization method: Keeping a calendar.

"Calendars help us see the opportunity cost of our decisions."-Dan Ariely, behavioral economist and psychologist

Maintaining a daily calendar makes you more aware of the small trade-offs you make every day, according to the psychologist. For example, is the free time you have after dinner best spent perusing social media or working on a personal project?

"It's all about opportunity cost, what you're giving up," says Ariely, whose book "Payoff" explores how to become more motivated. "Calendars help us see the opportunity cost of our decisions."